An image sensor may sense light emitted from a subject and convert the sensed light to an electrical value. The image sensors may be classified as charge coupled device (CCDs) image sensors and complementary metal oxide semiconductor (CMOS) image sensors.
The CCD image sensor may include MOS capacitors that store and transfer electric charge carriers. The CCD image sensor has drawbacks such as complicated driving method, high power consumption, and large number of mask steps. Accordingly, it is difficult to dispose a signal processing circuit inside a CCD chip.
The CMOS image sensor may include a plurality of unit pixels each including a photodiode (PD) and a MOS transistor. The CMOS image sensor may form an image by detecting a signal in a switching method.
The CMOS image sensor has advantages such as low manufacturing cost, low power consumption, and relatively easy integration with a peripheral circuit chip. The CMOS image sensor may be manufactured using the CMOS manufacturing technique. Accordingly, it is relatively easy to integrate the CMOS image sensor with a peripheral system that amplifies and processes signals. Due to easy integration, a manufacture cost thereof is relatively low. Further, a processing speed is fast, and power consumption is as low as about one percent of the CCD image sensor.
Pixels of an image sensor may be divided into a passive type and an active type. The passive type pixel has no buffer that serves as an active element. That is, a photodiode generates an electrical signal and transmits the generated signal along a vertical line when a horizontal selection signal is activated. Accordingly, the passive type pixel cannot perform a signal amplification function or a signal reduction minimization function. The active type pixel has a buffer that serves as an active element. The buffer has characteristics of high input impedance and low output impedance. Due to such characteristics of the buffer, signal reduction operation may be minimized even when an electrical signal is supplied with high output impedance. Further, a signal can be easily transferred without any loss even when a receiving terminal receives an electric signal with the input impedance.
A unit pixel of a typical passive type image sensor may include a photodiode, a transfer transistor, a reset transistor, a drive transistor, and a select transistor. The photodiode receives light and generates photocharges. The transfer transistor transfers the photocharges generated by the photodiode. The reset transistor sets a node potential to a desired value and emits the photocharges in order to perform a reset operation. The drive transistor serves as a source follower buffer amplifier. The select transistor performs addressing through switching.
A passive type image sensor, however, has disadvantages in which an output signal of the unit pixel is reduced in size by about 20% due to the body effect of the drive transistors.
In order to eliminate such a drawback, many studies have been made to exclude a drive transistor from a unit pixel of a passive type image sensor.
U.S. Pat. No. 6,975,356 discloses a passive type image sensor having no drive transistor.
FIG. 1 is a circuit diagram that illustrates a passive type image sensor of U.S. Pat. No. 6,975,356.
As illustrated in FIG. 1, the image sensor includes transfer gates 10, 12, 14, and 16 and photodiodes 2, 4, 6, and 8 in photosensitive sections of respective pixels. A source electrode of each transfer gate is respectively connected to a cathode of corresponding photodiode. The image sensor further includes source junction capacitors 18, 20, 22, and 24 of floating diffusion regions between drain electrodes of the transfer gates 10, 12, 14, and 16 and source electrodes of horizontal selection switches 26, 28, 30, and 32. Such source junction capacitors 18, 20, 22, and 24 are used as detection capacitance.
Gate electrodes of the transfer gates 10 and 12 are connected to a transfer gate control line 62. Gate electrodes of the transfer gates 14 and 16 are connected to a transfer gate control line 70. Gate electrodes of the horizontal selection switches 26 and 28 are connected to a horizontal selection line 64. Gate electrodes of the horizontal selection switches 30 and 32 are connected to a horizontal selection line 72.
The image sensor further includes reset switches 34, 36, 38, and 40 for charging the detection capacitors 18, 20, 22, and 24 to a reset level. The reset switches 34, 36, 38, and 40 are respectively connected to the detection capacitors 18, 20, 22, and 24. Drain electrodes of the reset switches 34 and 36 are connected to a reset voltage supply line 58 to which a reset voltage is supplied. Source electrodes of the reset switches 34 and 36 are respectively connected to the detection capacitors 18 and 20, and gate electrodes of the reset switches 34 and 36 are connected to a reset control signal line 60. Drain electrodes of the reset switches 38 and 40 are connected to a reset voltage supply line 66 to which the reset voltage is supplied. Source electrodes of the reset switches 38 and 40 are respectively connected to the detection capacitors 22 and 24. The gate electrodes of the reset switches 38 and 40 are connected to a reset control signal line 68.
The horizontal selection switches 26 and 30 are connected to a vertical selection line 54. The horizontal selection switches 28 and 32 are connected to a vertical selection line 56. Electric charge amplifiers 41 and 43 are respectively connected to the vertical selection lines 54 and 56.
The passive type image sensor of FIG. 1 may cause blooming effect. That is, the passive type image sensor performs light integration operation to integrate photocharges generated by the photodiodes 2, 4, 6, and 8. When the photocharges exceed a predetermined threshold value during the light integration operation, the photocharges leak through the transfer gates 10, 12, 14, and 16 and the horizontal selection switches 26, 28, 30, and 32. Such leaked photocharges adversely affect adjacent pixels and it causes a blooming effect.
FIG. 2 is a circuit diagram that illustrates an image sensor configured to prevent a blooming effect, and particularly, an active type image sensor disclosed in U.S. Pat. No. 7,385,272.
As illustrated in FIG. 2, a transfer gate 220 is disposed between a cathode of a photodiode 210 and a floating diffusion region 240. A gate of a source follower 260 and a current transport terminal of a reset transistor 230 are connected to the floating diffusion region 240. Another current transport terminal of the reset transistor 230 is coupled to a current transport terminal of the source follower 260. A voltage selection circuit 110 is coupled to the transfer gate 220.
The active type image sensor of FIG. 2 sets a voltage of the gate electrode of the transfer gate 220 to an intermediate value while the photodiode 210 integrates photocharges in order to prevent the blooming effect. Accordingly, the active type image sensor needs a voltage selection circuit 110. Such hardware requirement not only significantly increases a cost of the image sensor but also reduces a fill factor of the image sensor.